Slow Draining Sink except when AAV is removed...

If I untwist one of the Oatey Air Admittance valves under either the main kitchen sink or the island sink the sink drains normally. Otherwise I hear gurgling, usually in the island sink. I put a plastic bag over the AAV pipe and unscrewed the AAV, the bag filled with air when I ran the sink. Isn't an AAV supposed to allow air in, not out? What is the likely problem?

YOu are correct in that the AAV lets air in, but not out. That is your problem. Air is being trapped in the pipe, and since the air cannot be vented because of the AAV, the water either cannot drain, or cannot drain very fast.

mdistel,
If you don't hear a suctioning sound while the vent is in place...replace it.
Look for a "Studor" vent.
If the problem where positive pressure, you'd likely have overspill when the vent were removed.
You could do a simple test by putting your hand over the hole while the sink is draining.

He already stated that there is an AAV. Suction/vacuum speeds up drainage. Vents stop vacuum thus slowing drainage rate down to whatever the water's head pressure will generate. Positive pressure slows or stops drainage.

Thank you for all the replies. Both the island and the main sink are vented. I'll have to have a plumber come sank the lines, I tried and didn't get anywhere. Now I need to find a good plumber in the Cleveland area.